With a nifty 50 & a standard 68mm set of extension tubes you can get the focal point to actually inside the lens. The shorter the lens, the easier that is to do, eg a 35mm you can leave one of the tube segments off & still focus at 0".
As already mentioned, you do get in your own way a lot at these distances, so you could drop yet another segment off to be able to move back an inch or two, or see if a ring-light can fill enough [I use big video light panels at these kinds of distances, never tried ringlight]. Also at these kind of distances your DoF is going to be well under a millimetre, so you may end up needing a tripod & rail, or fast multiple captures, to get anything useful, as it's pretty hard to hold the camera that still.
BTW, the difference in DoF at such short ranges from wide to narrow aperture is minimal, so you'd do better to keep the aperture open so you can actually see properly. You really do lose a lot of light through this sort of setup [hence my video panels, so I can see what I'm supposed to be photographing through the lens.] Auto-focus also becomes useless, so switch it off. You can change focus by merely breathing, let alone consciously moving.
Some examples - starring the "Incredibly Interesting Pencil Eraser™️"
All shots are manually focussed to infinity so this is the maximum distance I can get back from the object, [very approximate] focusing by putting my hand between lens & object to try keep still. All shot at f/2. high ISO, 6400 so I can get below 1/500s exposure with my back to the window to get some light.
These are all very, very rough - 'scuse the 'fine art'.
I would normally do anything like this with controlled lighting & a tripod & rail.
Click for full size
50mm 1.4 with 32mm extension, distance approx 3"
50mm 1.4 with 68mm extension, distance approx 2"
35mm 1.8 with 32mm extension, distance approx 1"
35mm 1.8 with 68mm extension, distance 0" pencil touching lens frame
As you can see, from anything less than about 3" I'm completely in my own light.
For all set-ups, the out of focus distance at which there is nothing but blur is about 6 - 8". I didn't try to capture this. The background you can actually see is about 8 feet away, mirrored reflection of a square window.
I had a bit of spare time, so I thought I'd extrapolate this to really emphasise why I don't think wide angle is good for macro.
I don't do much wide angle so this is just using the kit lens at 18mm, f/3.5 with the ISO ramped up again.
18mm 3.5 with 12mm extension - the shortest I have - about 0.5" [any more extension & I can't focus outside the lens at all]
18mm 3.5 with no extension about 3"
Frankly, I think both make lousy pictures. Even allowing for the narrow aperture on the kit lens, the out of focus areas [still min 8 ft, & now out to 20ft or so, behind the subject] are far too clear. With the extension reducing the focus distance, I'm again right in my own light too.
My personal conclusion - use longer lenses & extensions to take macro pictures without a dedicated macro lens.
By way of contrast, these are shots taken using the same setup but in far more controlled conditions… the first is a single exposure of a lily, 3 or 4" across [single anther maybe 10mm], the second a 10-layer focus stack, allium bud, 2mm tall.
The backgrounds are done with ordinary coloured bath towels, 6-8 feet behind the subject & lit separately. The fades between colours are simply the blur at that distance, ie 2 different coloured towels becomes a vignette between two different colours.
At the other end of the scale, if you're trying to photograph anything that might spook & fly off, you should be looking at a 150mm or longer macro instead. They're just as tricky to hand-hold, even if stabilised, but at least your subject is less likely to vanish just as you're ready.
again, I am not looking for 1:1 reproduction ratio here, hence, the question.
– Harvey King Jan 03 '22 at 09:47